Observations on the voltage response of a 90° twisted nematic cell using guided modes

Using prism coupling to guided modes and surface plasmons we have examined in detail the director response of a 90° twisted nematic liquid crystal cell as a function of applied voltage. By careful comparison of angle scan reflectivity data with theoretical predictions generated from a combination of liquid crystal continuum theory and multilayer optics theory it has been possible to establish how the surface tilt changes with voltage, and also to observe changes in the optic constants due to changes in the order parameter with applied field.     

The director structure and electroclinic fast optical switching in a grating-coupled ferroelectric smectic liquid crystal cell

Optical excitation of guided modes in a liquid crystal layer using grating-coupling gives sharp features in the angle-dependent reflectivity data. These features are strengthened by using a metallized grating to enhance coupling to the guided modes in the liquid crystal. In the present study the liquid crystal has a smectic A phase which exhibits fast electroclinic switching. Combining the sharp features in the reflectivity together with the electroclinic effect leads to fast, high contrast, optical switching which may open up potential for novel device structures.     

The director structure and electroclinic fast optical switching in a grating-coupled ferroelectric smectic liquid crystal cell

Optical excitation of guided modes in a liquid crystal layer using grating-coupling gives sharp features in the angle-dependent reflectivity data. These features are strengthened by using a metallized grating to enhance coupling to the guided modes in the liquid crystal. In the present study the liquid crystal has a smectic A phase which exhibits fast electroclinic switching. Combining the sharp features in the reflectivity together with the electroclinic effect leads to fast, high contrast, optical switching which may open up potential for novel device structures.     

Momentum space imaging of a liquid crystal filled Fabry-Perot

Liquid crystal layer characteristics have for some time been determined by analysis of the optical guided modes supported within such layers. A novel technique for guided mode analysis, relying on scattering and re-radiation of light from a liquid crystal filled Fabry-Perot, is presented here and compared with a more standard technique, where the transmissivity of light through such a structure, as a function of the angle of incidence, is measured. The new technique which we label 'momentum space imaging' is found to hold some advantages over traditional methods, with data acquisition on the millisecond time-scale and full liquid crystal layer characterization being achievable with minor modification to existing theoretical multilayer modelling.     

New low viscosity liquid crystal compounds containing the 2,3,4-trifluorophenyl moiety for active matrix displays

The synthetic route and physical properties for new three-ring derivatives, trans -4- n -alkyl-4'-(2,3,4-trifluorophenyl)bicyclohexanes ( n CCHB3F) with alkyl chain length n from 2 to 5 are presented. They exhibit broad range nematic phases exceeding 40 K, and low viscosity with a low activation enthalpy; they also show low dielectric anisotropies and high birefringence. Their dielectric properties are compared with those of their three-ring homologues with a lateral -NCS group. The Maier-Meier theory is used to evaluate the temperature dependence of the value of 'dielectric anisotropy divided by order parameter ( Δ ε/ S )'. The temperature dependence of Δ ε/ S in several phenylbicyclohexane homologues is explained. Furthermore, the viscosity of nematic liquid crystal mixtures containing these materials is significantly reduced. These mixtures are suitable for the improvement of the switching time of liquid crystal displays using TFT Twisted Nematic (TN) modes.     

The optical tensor configuration in a surface stabilized ferroelectric liquid crystal determined by using half leaky guided modes

The optic tensor configuration in a surface stabilized ferroelectric liquid crystal cell is investigated using optical excitation of half leaky guided modes. A thin ferroelectric liquid crystal layer is confined between a high index pyramid, with an index greater than the maximum of the liquid crystal, and a glass substrate having an index less than the minimum of the liquid crystal. Using standard attenuated total reflection experimental procedures, over a small angle range a series of sharp resonant peaks are recorded in the s-polarized reflectivity using p-polarized incident light. These peaks are extraordinarily sensitive to details of the optical tensor configuration within the cell. Fitting theoretically modelled reflectivities from multilayer Fresnel theory to the data allows determination of near surface alignment, bending of the chevron, surface tilt angle and biaxiality. To give a clear physical explanation for the great sensitivity of the technique, the electromagnetic field component distributions in the cell are also presented and analysed. The results confirm that the half leaky guided mode method has enormous potential for the study of the optic tensor configuration in liquid crystal layers.     

Pretransitional electro-optic response of 6CB liquid crystal in the isotropic phase

In this work, the electro-optic response of a 6CB liquid crystal layer is studied using a sensitive differential technique. The layer is held at a temperature just above the nematic to isotropic phase transition. Transverse magnetic (p) polarized light incident on the cell is coupled to guided modes in the liquid crystal layer using prism coupling. The modes manifest themselves as sharp dips in the reflectivity as the angle of incidence is scanned. When a low frequency sinusoidal voltage is applied to the cell, the resonant mode shapes and excitation angles are altered at a frequency which is twice that of the applied field, resulting in a modulation of the reflectivity for a given angle of incidence. By synchronous observation of the modulated signal, a differential signal is recorded. Comparing the data with modelling generated from multilayer optics theory, two effects are then quantified. The first of these is an induced birefringence, varying quadratically with applied voltage, which is well understood and can be expressed in terms of Landau-de Gennes theory. The second is a field induced perturbation in the imaginary part of the optical permittivity, δε_i, which implies a modification of the light scattering properties of the liquid crystal. The measurement of the latter effect is, as far as we know, a novel one, being only made possible by the remarkable sensitivity of the synchronous differential technique.     

Use of mode mixing to determine the optic tensor configuration of a thin ferroelectric liquid crystal layer

It is possible to probe directly the optic tensor profile within a thin ferroelectric liquid crystal layer by the propagation of prism-coupled leaky guided modes. Plane parallel monochromatic radiation of single polarization is made incident through a prism into a planar ferroelectric liquid crystal layer. Monitoring the reflected signal as a function of incident angle results in a series of sharp dips at angles corresponding to the excitation of modes in the layer. For a ferroelectric layer in which there is in-plane or out-of-plane tilt, conversion of one linear polarization to the orthogonal polarization may occur, by detecting this conversion it has been possible to show the existence of thin boundary regions at the two surfaces of the thin ferroelectric liquid crystal layer.     

Pretransitional electro-optic response of 6CB liquid crystal in the isotropic phase

Abstract

In this work, the electro-optic response of a 6CB liquid crystal layer is studied using a sensitive differential technique. The layer is held at a temperature just above the nematic to isotropic phase transition. Transverse magnetic (p) polarized light incident on the cell is coupled to guided modes in the liquid crystal layer using prism coupling. The modes manifest themselves as sharp dips in the reflectivity as the angle of incidence is scanned. When a low frequency sinusoidal voltage is applied to the cell, the resonant mode shapes and excitation angles are altered at a frequency which is twice that of the applied field, resulting in a modulation of the reflectivity for a given angle of incidence. By synchronous observation of the modulated signal, a differential signal is recorded. Comparing the data with modelling generated from multilayer optics theory, two effects are then quantified. The first of these is an induced birefringence, varying quadratically with applied voltage, which is well understood and can be expressed in terms of Landau-de Gennes theory. The second is a field induced perturbation in the imaginary part of the optical permittivity, δε_i, which implies a modification of the light scattering properties of the liquid crystal. The measurement of the latter effect is, as far as we know, a novel one, being only made possible by the remarkable sensitivity of the synchronous differential technique.     

Use of mode mixing to determine the optic tensor configuration of a thin ferroelectric liquid crystal layer

Abstract

It is possible to probe directly the optic tensor profile within a thin ferroelectric liquid crystal layer by the propagation of prism-coupled leaky guided modes. Plane parallel monochromatic radiation of single polarization is made incident through a prism into a planar ferroelectric liquid crystal layer. Monitoring the reflected signal as a function of incident angle results in a series of sharp dips at angles corresponding to the excitation of modes in the layer. For a ferroelectric layer in which there is in-plane or out-of-plane tilt, conversion of one linear polarization to the orthogonal polarization may occur, by detecting this conversion it has been possible to show the existence of thin boundary regions at the two surfaces of the thin ferroelectric liquid crystal layer.     

Structural anisotropy study by resonance Raman scattering in nematics with different dyes

The method for the structural anisotropy determination, which is based on a comparison of the degree of polarization of absorption and resonance Raman bands, is tested with three dyes having different molecular dimensions in one nematic liquid crystal. The experimentally observed correlation between the variation of the structural anisotropy factor and the order parameters deviates from the predictions of the Segre model. This method could also be applied to determine the local field anisotropy for liquid crystals, but we must take into account the observed dependence of the structural anisotropy on the dimensions of the probe molecules.     

Characterization of the alignment of a nematic liquid crystal using half leaky guided modes

The excitation of half leaky guided optical modes to characterize fully the optical tensor profile in a thin liquid crystal layer has been used to evaluate the effect of rubbed polyimide aligning layers on the alignment of a nematic liquid crystal. A cell fabricated with rubbed polyimide alignment surfaces was studied at a wavelength of 632.8 nm. The liquid crystalline layer is sandwiched between a high refractive index top glass plate and a low refractive index glass substrate. Angular dependent reflectivities are recorded using a coupling prism and matching fluid with the same index as the top glass plate. Careful fitting of the predictions from multilayer optics theory to the observed angle dependent polarization conversion and reflectivity data yields the director profile within the liquid crystal layer in great detail.     

Dynamic processes in ferroelectric liquid crystal filled cells

The switching process in a ferroelectric liquid crystal cell is of great interest. The precise way in which the optic tensor structure reorientates during switching between states is, however, difficult to determine. Here we consider the use of guided modes and surface plasmon-polaritons as techniques for the investigation of this. It is seen that because of the nature of the dynamic processes the guided mode data is inconclusive, but surface plasmon-polariton data show the surface reorientation mechanism.     

Experimental investigation of the compressible continuum theory of a homeotropically aligned ferroelectric liquid crystal

The optical tensor configuration in a homeotropically aligned ferroelectric liquid crystal (FLC), SCE13 cell, is investigated by means of optical excitation of half leaky guided modes. A thin slab of FLC is confined between a high index pyramid and a low index substrate whose indices bound those of the liquid crystal. In this geometry there exists a small angle range over which a series of sharp resonant modes may propagate in the liquid crystal. Detecting the angular dependent reflectivity for plane polarized radiation and subsequently fitting the data by iteratively modelling from multilayer Fresnel theory, a full characterization of the tilt and twist profile throughout the cell is achieved. The temperature dependence of the tilt of the principal director, which is related to the smectic cone angle, and of the optical permittivity, as well as the pitch have been obtained. The tilt director profile across the cell is interpreted using a compressible continuum theory for SmC* liquid crystals which includes the possibility of variable cone angle and layer spacing.     

A study of the electroclinic effect using half-leaky guided modes with a homeotropically aligned liquid crystal

High resolution voltage dependent tilt angle studies using optical excitation of half-leaky guided modes have been conducted on a homeotropically aligned ferroelectric liquid crystal mixture (Merck SCE13) in the S_A phase. Uniform homeotropic alignment is realised, with no surface aligning layer, by the application of an in-plane DC electric field when the liquid crystal is in the S_C* phase. The applied field unwinds the pitch of the S_C* chiral helix and gives a uniformly tilted homeotropic monodomain. On warming into the S_A phase, detailed studies of the voltage induced tilt, the electroclinic effect, are then conducted at various temperatures. Because there is no influence of surface anchoring forces, the linear relationship between the induced tilt angle and the DC field is obtained even under very weak fields. Further, the relationship between induced tilt and temperature confirms the predictions of a second order Landau mean-field theory with a coupling term between the tilt angle and the DC field.     

Stability of liquid crystal micro-droplets based optical microresonators

Liquid crystal (LC) based tuneable optical microresonators are potential for being used as crucial components in photonic devices. In this article, we report experimental studies on LC micro-droplets dispersed in several dispersing media. We find that the size of the micro-droplets formed in a low refractive index and optically transparent perfluoropolymer are most stable with time than commonly used dispersing media. Using a negative dielectric anisotropy nematic liquid crystal, we show that the whispering gallery mode optical resonance properties such as the quality factor and the free spectral range of stable micro-droplets are independent of the strength of the applied electric field. The optical resonance properties under applied field are significantly different than that of the liquid crystals with positive dielectric anisotropy and are explained based on the elastic deformation of the micro-droplets.     

Microtubule particle dispersion in liquid crystal hosts

Microtubule particles and metal-coated microtubules were dispersed in various host liquid crystal mixtures. Dispersion effects were evaluated as a function of liquid crystal type, viscosity, dielectric anisotropy and surface interaction. Experimental results indicated that all the types of liquid crystals studied were aligned perpendicular to the microtubule surfaces, regardless of liquid crystal composition or various surface coatings used on the metal-coated microtubules. Low concentrations of the metal-coated microtubules in nematic liquid crystal hosts were aligned by flow or cell surface alignment conditions, and could be modulated by electric or magnetic fields. We observed better microtubules dispersion uniformity in high viscosity liquid crystal host mixtures and in liquid crystal-monomers than in isotropic fluids. Microtubules particles dispersed in ROTN-404 liquid crystal mixture had a much higher birefringence in the microwave region than dispersion in a paraffin oil.     

Dielectric studies of tetraethylene glycol-bis(3-methylimidazolium) dichloride (TEGDC) exhibiting large negative dielectric anisotropy

A dielectric anisotropy property of a TEGDC (tetraethylene glycol-bis(3-methylimidazolium) dichloride) is investigated as a function of frequency. TEGDC showed an extremely large negative dielectric anisotropy (-10.95 to -4753.73). Variation of dielectric anisotropy (delta epsilon) with respect to the spot frequencies reveals that liquid crystal (LC) orientation has an n-type property at low frequencies and as the frequency increases dielectric anisotropy character shifts from negative dielectric anisotropy type (n-type) to positive dielectric anisotropy type (p-type). Consequently, the TEGDC is a liquid crystal with large negative dielectric anisotropy.     

Evaluation of optical anisotropy in the pretransitional regime in antiferroelectric liquid crystals

By measuring the transmission through an antiferroelectric liquid crystal device (placed between crossed polarizers) as a function of both orientation and applied field in the so-called electroclinic-like, pretransitional regime the behaviour of the effective optic axis tilt angle (psi) and effective optic anisotropy (Delta n ) is determined. The relatively poor alignment obtained in antiferroelectric liquid crystal devices is allowed for in the data interpretation and high quality results are presented.     

Dynamic processes in ferroelectric liquid crystal filled cells

Abstract

The switching process in a ferroelectric liquid crystal cell is of great interest. The precise way in which the optic tensor structure reorientates during switching between states is, however, difficult to determine. Here we consider the use of guided modes and surface plasmon-polaritons as techniques for the investigation of this. It is seen that because of the nature of the dynamic processes the guided mode data is inconclusive, but surface plasmon-polariton data show the surface reorientation mechanism.